High fidelity radio receiver



Dec. 27, 1938. P. F. G. HOLST ET AL f 2,141,391

vHIGH FIDELITY RADIO REcEIyER Filed Aug. 29, 1936 2 Sheets-Sheet 2 Patented Dec. Z7, 1938 HIGH FIDELITY RADIO RECEIVER Poul F. G. Holst, Oaklyn, and Loren R. Kirlr- Wood, Collingswood, N. J., assignors to Radio Corporation ofV America, a corporation of Delaf i Ware Application August 29, 1936, Serial No.' 98,443

13 Claims.

The present invention relates to high fidelity radio receivers of the type having variable selectivity control means whereby the audioy frequency response therefrom may be extended in range in response to an increase in the strength of a Vreceived carrier wave.

An object of the present invention is to provide an improved high iidelity radio receiving system having an audio frequency amplifier which is automatically controlled to receive signals through two, receiving channels having diiering degrees of selectivity, or either of said channels selectively, and an improved control system for a multiple channel receiving system of that character.

tion to provide an improved automatic volume control and suppression system for a radiore' ceiver in conjunction With selectivity control v means,v whereby the receiver is responsive to relatively strong modulated carrier waves with an extended audio frequency range of response. ,It is a still further object of the present invention to provide a superheterodyne receiver having a plurality of intermediate frequency amplifying 2liA channels terminating in a common audio frequency amplifier, with an improved control means therefor.

It may be considered as a further object ofthe invention, also, to provide a superheterodyne radio receiving system having two intermediate frequency amplifier channels of differing selectivity and gain, and automatic means responsive to carrier wave strength for passing signals through either of said channels in accordance with thev carrier wave strength.

It is also an'object of the invention, to provide a radio receiver with automatic variable selectivity control means at a moderate cost comparable with the results obtainable, and an improved and eifective sensitivity control means therefor.

Other objects and features of the invention will be apparent from the following description wheny It is also a further object of the present invenf (Cl. Z50- 20) operating characteristics of the system of Figures 1 and 2.

Referring to Fig. 1, a radio receiving system is shown comprising a sharp channel and a broad channel both of which are jointly controlled by a control channel as designated on the drawings. The main receiving channel comprises a radio frequency amplifier I, coupled to a first detector 2 which supplies intermediate frequency signals to first and second stage intermediate frequency amplifiers, 3 and A respectively. The latter is followed in the receiving channel by a second detector 5, and a suitable audio frequency amplifier comprising `first and second stages 6 and I respectively. A loud speaker or suitable output device is provided as indicated at la. An oscillator S is connected with the rst detector to change recivedsignals to the desired intermediate frequency.

The main signal channel, above described, is relatively sharply tuned to respond to relatively Weak signals with a high degree of selectivity in the intermediate frequency amplifier stages 3 and 4; and the second detector 5.

A broadly tuned intermediate frequency amplierand detector are also provided as a second signal channel in the receiver, and comprises an amplifier stage 9 and a second detector I0 coupled to the output circuitAof the rst detector. The second detector I0 is arranged to apply rectified signals to the audio frequency amplifier and a control potential to the second intermediate frequency amplifier stage, as indicated by the schematic circuit connections.

AA further connection is madewith the first detector output circuit for the control channel comprisingr anV intermediate frequency amplier stage I I towhich is coupled automatic volume control meansV I2, I3 and I5 in parallel, the automatic volume control means I3 also being connected With a suppresser stage I4.

The suppresser stage I4 is connected with the intermediate frequency amplifier stage 9 of the broad channel and also the automatic volume control means I is ,connected to the same stage for joint control of the broad channel I. F. amplifier.

The automatic volume control means I2 is arranged to -control the gain in the radio frequency first detector andthe first intermediate frequency amplifier stages of the sharp channel as indicated.

The operation of the system is such that for Weak carrier Waves, the main sharp signal channel is utilized to'apply the signal output to the audio frequency amplifier and loud speaker while the broad channel is suppressed and inoperative,

and as the strength of a received carrier wave increases or a strong carrier wave is received, the sharp channel comprising the intermediate frequency amplifiers 3 and 4 and the second detector 5 is gradually cut off and the broad channel, comprising the intermediate frequency amplifier 9 and the second detector lll is placed in operation.

In the first instance, the automatic volume contrcl potentials are provided bythe device I2 for the sharp channel, while in the second instance the automatic volume control potentialsl are supplied by the device I5 to the I. F. amplifier 9.

It will thus be seen that there is provided in the receiver, a modulated carrier wave or intermediate frequency amplifier having an amplifying and detecting channel providing a high degree of selectivity and a corresponding lesser degree of fidelity, and a broadly responsive intermediate frequency amplifier channel providing a high degree of delity. The control channel, also operating in parallel Awith the first two channels at the same intermediate frequency, is arranged for differentially controlling the first two channels, that is, to control the change over operation as above described, from one channel to the other through a series of automatic volume control and suppressor means provided with a common intermediate frequency amplifier whereby effective control potentials are available. The operation of the system, and the sensitivity control means provided in conjunction with the control channel, will further be understood by reference to Fig. 2 wherein the circuit details are shown for the three channels in the intermediate frequency amplifier and the first stage of the audio frequency amplier, which operates in conjunction therewith.

Referring now to Fig. 2, wherein the like reference numerals are applied to the same amplifier and control stages `as in Fig. l, the intermediate frequency output from theV first detector is applied to the first intermediate frequency amplifier tube 3 through a suitable tuned intermediate frequency input transformer 20. The tube 3 is ooupledthrough a second intermediate frequency coupling transformer V.2l to the second intermediate frequency amplifier 4.v The-second stage intermediate frequency amplier is coupled to a second detector 5. The detector is of the diode rectifier type having Va cathode 22, an anode 23, and'further includes amplifier elements comprising the common cathode, a control grid 24 and an output anode 25. 1

The coupling between the intermediate fre-l quency amplifier 4 and the detector 5 is provided by a tuned intermediate frequency output coupling transformer 25 providing a tuned secondary circuit 2l which is connected between the diode anode 23 and the cathode 22 through a diode output resistor 28. The latter is provided with a suitable intermediate frequency bypass capacitor V29. Thecathode is connected to ground or the common circuit return system of the receiver as indicated at 30, while the negative terminal 3| of the output resistor 28 is connected through an output lead 32, a coupling resistor 33, and a grid lead 34, withthe control grid 24. 'Y Y 'Ihe audio frequency potentials established across the diode `output resistor 28 Vare thereby applied to the control grid 24 and kare Vsuitably Vamplified in the amplifier ,section thereof comprising the cathode 22, the control grid 24 and the main output anode 25. The latter is couplied to the audio frequency amplifier through an output couplingimpedance 3B and a` coupling capacitor 31. With this arrangement, the detector and first audio frequency amplifier are combined in one device although it should be understood that they may be provided in separate stages as schematically represented in Fig. 1. Accordingly, the first audio frequency amplifier is designated in the device 5 by the reference numeral 6 as used in Fig. 1.

The intermediate frequency coupling devices 2U, 2| and 26 represent the usual selective intermediate frequency coupling devices. Such coupling devices are represented in the present eX- ample, by transformers having tuned primary and 4secondary windings suitably coupled to provide a predetermined high degree of selectivity Without appreciably attenuating the desired signal wave side bands.

The broad channel, however, is more fully responsive to the carrier wave and side bands without appreciable attenuation, to permit audio frequency fidelity substantially to the limit of a desired band such as to 8 kilocycles, for example.

In this channel, the first stage intermediate frequency amplifier 9 is .provided with a grid coupling Y.resistor 40 connected with the grid potential supply lead 4i and through a coupling capacitor 42 with abroadly tuned intermediate frequency input circuit 43 which is coupled vinductively as indicated, with the intermediate frequency input transformer 25 through a coupling winding 44. Any other ksuitable coupling means for the device 9 may be employed in conjunction with the main signal channel .from the first detector.

The yamplifier Ydevice 9 is coupled on its output side, through a broadly tuned intermediate fre- .quency coupling transformer 45 with the diode second detector l in the broad channel. This comprises a rectifier device having a cathode 4'6 and an anode electrode 41 connected with the tuned output circuit 48 of the transformer 45 through .a diode output resistor indicated at 49. In this circuit, the cathode 45 is also connected to ground or the common chassisV return circuit, as indicated at 50. In the present example atriode device' is 'utilized asa rectifier, the anoder41 being a normal grid electrode and the mainY anode I being connected to the cathode 45. A suitable intermediate frequency bypass capacitor 52 is provided Yacross thediode output resistor 49. The detector lll is connected, in with the detector..5, .to the .rst audio frequency amplifier stage 6, through a similar coupling arrangement as provided for the detector 5. This comprises an audio frequency output lead 53 connected with the negative terminal 54 of the diode output resistor 49 and connected further through a high'resistance element V55 corresponding to the resistor 33, to the lead 34 at a junction point 55 between the two resistors. Y

Withthis arrangement it will be seen that modulated signals received from the first detector, may be amplified through either or both of the amplifying channels, and afterfbeing rectified in either or both of the detectors 5 and l0, the audio frequency component is applied through the output leads 32 and 53 andthe resistance elements 33 and 55 to the grid of the first audio frequency amplier'. Y Y

The latter grid may also receive biasing potential from both circuits with the signal potentials v Y therefrom, whereby the sharp channelrmay be effective to provide a normal audio frequency response, or the higher fidelity response may be eife'cted through the more broadly tuned chan- Aeo nel, without changing any of the circuit connections.

The series resistors 33 and 55 between the output resistors 28 and 49 are preferably relatively highin value whereby the two diode circuits are substantially de-coupled. Thisv is preferable in order to keep the interference between the two channels relatively low and in. order to do this, the ratio of the resistors 33 or 55 to the resistors 28 or 49 should be kept as large as practically possible.

Either of the two signal channels may separately be connected with the audio frequency amplifier by operation of a simple two-point switch having a movable contact 58 connected to ground 51 and having two contact points 58 and 59 connected with the audio frequency output leads 32 and 53, respectively, for the two audio frequency channels.

Closure of the switch arm 56 with the contact 58 serves to ground the'output lead 32 and cuts off. the audio frequency output from the sharp channel, whereby signals are transmitted from the broad channel through the lead 53 and the lead 34 from the tap 55 between the two resistor sections 33 and 55, thereby providing, in effect, a tap midway of a resistor comprising the .two sections 33 and 55. One-half of the available output potential on the lead 53 is applied tothe lead 34.

In a similar manner, when the switch arm 56 is connected with the contact 59, the lead 53 is grounded, whereby signals :are taken from the sharp channel through the lead 32 and the lead 34 from the tap 56, which again serves as a midtap on the resistor 33--55.

Through the medium of a simple two-point switch' having a mid position, signals may be taken from either or both signal channels and applied to a common audio frequency amplifier grid circuit. The vswitching arrangement is further simplied by the fact that no additional biasing potential is necessary other than that provided by the incoming signal, as derived from the diode output resistor or resistors. Y

This channel mixing system is shown, described and claimed in our Patent No. 2,127,816, issued August 23, 1938, for Multichannel radio receiving systems, and assigned to the same assignee as this application. Accordingly, further description of this portionof the system is believed to be unnecessary.

Gain or volume control potential for suppression in the sharp channel and, in the present example, for the second intermediate frequency Y amplifier stage 4, is derived from the second detector of the broad channel. A volume control lead V5I) for the stage 4 which includes a lter comprising a series resistor 6I and bypass capacitor 32, is connected with the negative output terminal of the diode output resistor 49. vThe lead 58 is connected through the an inductor 55 and the grid lead 65, to the control grid 61. The cathode 38 is connected to ground 69 through a self-bias resistor 18 provided with a bypass capacitor 1I.

I'hisarrangement serves tovplace afxed biasing potential of predetermined value on the grid 51, from the self-bias source 10, and an additional,

biasing potential, in the presence of signals, as derived from the detector output resistor, 49. This operates to reduce the gain-of the stage 4 and thereby to regulate the volume r signal level inthe sharp channel. The potential derived from this control source'is, on strong signals, sufficient to fully suppress the sharp channel.

tuned input circuit 64' "first intermediate frequency` amplifier 3.

AutomaticV volume control potentials for the radio frequency amplifier, as diagrammatically indicated in Fig. 1, are derived from the automatic volume control rectifier'.l l2, which is preceded by an automatic volume control intermediate frequency amplifier |I. In the present eX- ample, the intermediate frequency amplifiery I I is a double diode pentode device containing the diode electrodes I2 and I5 for the diode devices designated in Fig. 1 by the same numerals', and.

having a common cathode 15 connected with a suppressor grid 16 and further having a control grid 11, ascreen grid 18 and an output anode electrode 19. f

Signalsat intermediate frequency are applied to the control grid 11'through a lead 89 which is coupled with the same input circuit 43 as is provided for the amplifier device 9 in the broad channel. In this case, the lead 80 is connected directly through the tuning inductance 44 of the circuit 43 and receives biasing potential therethrough from a biasing potential supply lead 8| which is connected with a tap point 82 on a series'resistor comprising two sections 83 and 84 between ground 85 and the cathode 15 of the automatic volume control amplifier Il. The resistors 83 and 84 are provided with suitable bypass capacitors 96, 81 for intermediate frequency currents.

The initial biasing potential applied to the grid 11 with respect to the cathode 15 is a normally self-'biasing potential obtainedY by the drop in the section 83.

The output anode 19 of the amplifier II is connected to the primary winding 98 of a natural period intermediate frequency amplifier coupling transformer 9|, the secondary 92 of which is connected at one end with the diode anode I2 and through a coupling capacitor 93 with the diode anode I5, and also through the coupling capacitor 93, a lead 94, a coupling capacitor 95 and a lead 96 to the anode electrode 91 of the rectifier I3 which, in the present example, is a triode device. The grid of the latter serves as the anode three diode rectifier devices comprising the anodes I2, I and 91. When the signal voltage is of suicient positive value to overcome the voltage existing on the cathode 15 the intermediate frequency signals are rectified andthe output therefrom is ,applied to an output resistor Isl through a series circuit which may be traced from the cathode 15, through the resistors 33 and 84, to ground 85 and a ground |92 for the resistor |0I, thence through the resistor IiI to a terminal |83, and from the terminal i193 to the secondary 92 and the anode I2.

The rectified signal current is of relatively low value with respect to the anodecurrent of the device II flowing in the resistors S3 and 84, and therefore does not serve to change the potential to any appreciable degree. Howeven in the output resistor IDI, a potential having a polarity as indicated is s et up and may be derived from the terminal |93 for negative biasing purposes and also from a suitable tap |04. The latter is connected through a bias supply lead |95 with the connection includes a filter comprising a series The resistor |01 and bypass capacitor |08, the secondary |09 of the transformer 20, a grid lead I|0 and the control grid of the amplifier 3. 'I'he automatic volume control potentials for the other input circuits of the receiver are taken from the other negative terminal ||03 on the resistor IOI through a lead 89-89 which is broken to simplify the circuit diagram.

The cathode I|2 of the intermediate frequency amplifier 3 is provided with a self-bias resistor ||3 and suitable bypass capacitor IM with a ground connection as indicated at |I5 to establish on the grid I I2 an initial negative bias for normal or full amplification of received signals.

With this arrangement, the preceding signal input stages and circuits of the receiving system, represented by the rst intermediate frequency amplifier, may receive additional automatic volume control or biasing potentials from the .output resistor |0| underY control of the diode I2.

It has been found that in connection With the main gain controlling circuits of the receiving system, such as above described, there may also be arranged a manually variable sensitivity or volume control means for the receiving system, without the addition of complicated circuits'and with only a limited number `of additional control elements. i 't It has been seen that the present automatic gain control system involves the operation of the diode rectier l2 to provide with increasing carrier wave or signal strength, an increasing'negative potential across the diode output resistor |0| and a corresponding increasing negative potential with respect to ground vat the point or terminal |03. A portion of this voltage is utilized for automatic volume control rpurposes through the lead |05 on rst I. F. amplifier 3, and the full voltage is used for automatic volume control purposes on the R. F. amplifier and rst detector, throughV the lead Btl- 89.

If, now, a potential drop is provided in a potentiometer device IIS having a polarity as indicated, with the positive end thereof |I`| connectedV to ground as indicated at |I8, the potentiometer contact II9 Vmay be connected through a lead |20 and a series potential drop regulating resistor |2| with the terminal |03, to supply an adjustable potential to the automatic volume control resistor |0|.

If the resistor I 2| has a relatively high resistance with respect to the output resistor IBI and the Contact IIS is movedto the terminal II'I, the operation of the automatic volume" control means is the same as previously described and the only delay on'the'diode I2 will be that set up by the drop in the self-bias resistor sec-n tions 83 and 84. Y y

VHowever, if the Contact I|9 vis moved along the source H3 to establish between ground and the terminal |03 a negative potential, this Will appear as a delay potential on the anode l2 and the potential Will be applied through the resistor |2| across the diode output resistor IlflI, and as the contact is moved toward a negative terminal indicated at |22, the full potential available will be applied across" the output resistor |0I, and the negative bias applied to the diode" anode I2 will be the potential derived Vfrom the resistor I I6 asV proportioned between the resistors |2| and IOI;

In the absence cfa received signal, itlwill be seen that by the adjustment ofthe contact H0,Y the sensitivity of the input circuits of the receiver may be limited to a desired degree-inf exactly the Ysame manner as if av signal were limited by reason of operation of the automatic volume control rectier I2,'since the negative potentials available on the terminals I 03 will be in the same proportion as if they were provided by a rectified signalon the anodeV I2.

However, this portion of the present system, comprising the sensitivity and noise suppression control of the diode I2, as above described brieiiy, is more fully shown and described and is claimed in our copending application, Serial No. 98,746, led August 3l, 1936, for Noise suppresser systems, and is assignedto the same assignee as this application.` Accordingly, this portion of the system will not further b-e described. Y

It should be noted that at the same time the sensitivity is reduced, a further delay potential is impressed upon the diode anode I2, causing the diode to become inoperative` until the signalV amplitude'is such that it corresponds to the gain predetermined by the adjustment of the sensitivity control contact H0. For this and higher signal strengths, the automatic volume control means, as described, functions in the ,same manner Yas it would if the sensitivity control were set for maximum sensitivity at the contact point I I l.

The broad signal amplifying channel comprising the intermediate frequency amplifier S and the detector: I0 is similar to the sharp channel except thatV the gain is preferably lower as indicated by the lesser number of stages in the present example, because of the fact that the broad channel is .used only in response to relatively strong carrier waves. Therefore, it is necessary that the automatic volume control means which controls the broad channel be provided with a greater degree of delay than the sharp channel inorder to Vobtain substantially constant audio frequency output on the first audio frequency amplifier as is desired, regardless of the signal strength.

The operation of the system is such that as the carrier Wave strength varies on the receiving system, a substantially constant audio frequency potential is delivered to the first audio frequency input grid E for the same degrees of modulation. This is accomplished by utilizing an independent diode I5 Afor the automatic volume potential supply for the broad channel intermediate frequency amplifier, and by returning the diode output resistor circuit to aV point on a potential supply system at a point more negative than ground, to which point the cathode or cathodes of the broad Vintermediate frequency amplifier channel are returned. i t. ,g v For the above control arrangement of they broad channel, ther/'grid bias supply lead QI of Vthe amplier 9 is connected through a potential drop producing resistor-|25 With a tap |26 on a diode" output resistor |21 connectedrin circuit with the diode anode I5 and the cathode 75. VThe connection is completed through a lead |28 and a source of negative biasing potential |29 to ground |58 and the ground 85, returning through the resistors 83 and 8@ to the cathodel.

Intermediate frequency signals supplied by the transformer 9| from the amplier are applied to the diodem|5 through the coupling capacitor 03, as described, and av portion of the negative potential developed across the output resistorV |27 is applied-to the control grid |30 of the amplifier 9, through the grid resistor 40, the supply lead QI andthe series resistor |25, from thek tap |20. i The cathode of the device 9 designated atVIBI, is connected through a self-bias resistor |32, having a' connection with vthe lead |28, whereby the potential applied between the grid |30 and the vcathode I3I may be provided by the self-bias resistorv |32 when the received signal strength is not suicient to overcome the delay on the rectiiier anode l5, from the sources |29 and 83-84, and as the signal strength increases to a value above the delay potential; the potential derived across the resistor |21 is added thereto to provid-e Aari increasing negative bias on the gridY |30 with respect to the cathode, thereby to reduce the gain in the broad channel, for automatic volume control purposes.

The diode I5, however, is not permitted to come into action until a predetermined signal strength is available, dependent upon the additional delay potential provided by the source |29 which is in series between the anode I and the cathode 15.

It will be seen that the peak amplitude of the signal delivered to the diode anode I5 musthave a positive value greater than the negative Vpotential available across the resistor |29 and the self-bias resistors |33 and 84, and the value of th Y delay potential may be adjusted to any suitable bias potential supply elements 1n the negative with increased signal on the broad channel, thereby providing a proper balance in output to deliver to the rst audio frequency amplifier grid 6 substantially a constant signal, as the transition is made from sharpchannel to broad channel receiving conditions.

The biasing potential source provided by the resistor II6, providing a negative potential supply for the contact II9 and the automatic volumev control diode I2, for sensitivity control, of theV receiver, and additional delayon the diode anode I2, and the potential source provided by the resistor Y|29 for the diode anode I5 may be con-V nected in any suitable manner to provide negative delay potentials with respect to ground IIB.

vIn the present example, resistors are connected in the negative power supply lead |40 for the receiver, which is connected withv the chassis or ground I|8 through a choke coil or eld winding I4! and the resistor |29, in series.

The resistor or potentiometer IIB is then connected between ground and the negativev terminal of the choke coil |4I through a lter resistor |42 provided withy a bypass capacitor |43,

whereby the resistors I 42 and the potentiometer H6 are connected in series as a parallel path across the choke coil |4I and the resistor |29.

This arrangement provides two separate negative lead of the power supply circuit for the receiver,

and is thereby adapted for the circuits abovev de-A scribed to supply biasing potentials thereto.

The difference between the potential drop through the resistor |29 and through theresistor ||6 is also applied across a potentiometer resistor |44 which is connected in parallel with said re-A sistorsbetween the terminal |34 on the lead |28 and acathodelead |45 for the suppressor tube I4 through a movable tap |45 which is connected l with'the-lead |00 for the cathode 98. This circuit arrangement provides a variable negative delay potential on the anode electrode 91 with respect to the cathode 98.

With no signals received through the input circuit 43and the amplifier I I, the signals applied tothe diode anode 91 are Zero, with the result that there is no biasing potential applied to the control grid |41, with respect to the cathode |46. The impedance of the suppressor tube l! 4 is thereby relatively low and the potential existing across the terminals of the potential supply source |44 causes current to flow through the resistor |21 and the resistor |25, to apply a cut oif potential to the amplifier tube 9 inthe broad channel, thereby, in the absence of received signals, serving to cut oif the broad channel response.

If the suppressor tube I4 is out off by applying a strong negative biasing potential between the control grid |41 and the cathode |46, the auto-v matic volume control system inY connection withv the diode l5 will function to control the amplifier 9 in a normal manner, as hereinbefore described.'Y

For this purpose, the diode rectifier I3 is provided with a variable delay potential Eb between the potentiometer contact |45 and the terminal |52. Thispotential is adjusted to a value such that when the carrier Wave or intermediate frequency signal strength increases, no current is drawn through the diode output resistor I5| until a predetermined high valuey ofthe carrierstrength is obtained. Up to this point, the broad intermediate frequency amplifier channel remainscut oif.

Beyond that point, when theV signal reaches a higher magnitude, the peak intermediate frequency potential is applied from the circuit lead 94 through the capacitor 95 to the diode I3, producing a Vnegative biasing potential across the outputresistor I5| and on the control grid I 41 of the suppressor tube I4 to out off the latter tube and to permit Vthe automatic volume control potential developed across the bias resistor |21 to control the grid |30 of the amplifier tube 9,A

Thereafter, the broad channel is controlled by the automaticvolume control means, comprising the diode anode I5.

By adjusting the contact |45 to vary the delay potential on the diode rectier controlling the suppressor tube, a variable delay in the operation of the broad channel may be provided.

The operation of the system briefly is as follows and may be considered by referring tor both Figs. l and 2: Y

A signal impressed upon'vthe input circuit of the receiver is amplified in the radio frequency amplifier and is converted to a predetermined intermediate frequency by the oscillator and first detector and may then flow through three channels, comprising the main intermediate frequency channel, including thetubes 3, 4 and 5, the broad intermediate frequency amplifying channel including the tubes 9 and I0, and through a control channel comprising the amplifier I I and the diodes I2, I5 and I3, the latter controlling a suppressor tube I4. I

The broad channel remains cut off, provided the signal is not strong enough to cut off the suppressor tube I4. The sharp intermediate frequency channel is controlledabove a predetermined delay, by the diode I2, and upon increase of signal strength to the'point where the suppressor cuts olf, the radio frequency signal strength is maintained constanten the sharp detector, comprising the diode 23 in the tube 5.

Increasing the signal strength further causes the broad channel to operate and to amplify signals, and the voltage rectified by the detector I0 will begin to suppress the signals in .the sharp channel by applying an automatic volume control potential to the second intermediate frequency amplifier tube 4. If the signal or carrierV wave still further increases in strength, the sharp channel finally may become entirely cut off by action of the volume control potential derived from the detector IB which is, therefore, effectively a suppresser tube for the sharp channel.

It is considered desirable, in order to .provide proper selectivity for the system, that the bro-ad channel may be entirely cut off in the presence of weak signals because even a'small portion of the signal coming through the broad channel at thatY time would definitely cause the'selectivity to be Ybelowthe desired permissible value, in which case the signals are received entirely through the sharp channel.

The Vresponse of the broad channel without the suppressor may be represented by the curve shown in Fig. 4 at ISU, and the addition of the suppressor modifies the action of Vthe broad chan- Y nel as represented by the curves ISIS., ISIb, and

I6Ic, the cut 01T points being extended to operate in response to a stronger signal by adjustment of the contact |45 to apply a high delay potential to the diode rectifier which controls the sup'- pressor tube. Thus, the advantage of providing a variable suppressor cut off is realized. 1

If now the direct current voltage created by the broad channel second detector IU is applied as an automatic control voltage on the second interme-diate frequency amplifier,A as described,

the overall response curve of`voltages applied to the audio frequency amplifier represented by the curve |62 may be obtained and results from the smooth change-over of the audio frequency amplifier from the sharp channel detector output to the broad channel detector output under control of the broad channel detector I0.

If the suppressor control is adjusted to provide the operating curves IGIa., Itis, or Ilc, the signal system may vary. It is evident that the signal or carrier wave strength at which the changeover takes place may be varied by changing the suppressor cut off in the broad channel and that the change-over from one channel to the other may be effected gradually by proper adjustment of the control contacts and I I9.

The system described has the advantage that more than one intermediate frequency channel is utilized to carry the modulated carrier Wave or signal and the two channelsare controlled simultaneously by a third control channel including a suppressor and separate automatic volume control means therefor, including a sensitivity control by which the initial sensitivity of the system may be adjusted, and may be operative to control the sensitivity up to a predetermined point at which the automatic volume control means begins to function.

The system further includes a control circuit which permits the change of operation from one channel to the other to be effected at a predetermined'signal level and includes a diode rectier circuit therefor and a Vmixing system lfor the two signal channels which requires no switching operation to change from one channel'to the other in connection with a common audio free quency amplifier.

The last named circuit has theY further advan tage that the audio frequency amplifier is diode or signal biased by the signalffrom the diode second detectors in both channels, either jointly or separately, depending upon the operation of the system through theY one or the other, or both intermediate frequency amplifying channels, thereby preventing overloading.

The use of a plurality ofV signal amplifying channels, such as three intermediate frequency amplifying channels in a radio receiving system, with one channel providing a control potential for the others, is shown, described and claimed in a copending application of Paul Weathers, for High fidelity receiving systems, led August 2,9, 1936, SerialNo. 98,435, and assigned to the sam assignee as this application.

Accordingly, the use of three signal amplifying channels, per se, are not claimed herein, but in all other respects, the invention should be 'understood as being not limited to the specific receiving system shown, and'only by the appended claims.

We claim as our invention:v Y

1. In a radio receiving system having a pair of signal amplifying channels offering .differing degrees of selectivity to received signals, the cornbination of demodulator means for each of said channels, means providing a gain controlling connection for deriving a gain controlling potential from onlyV Vone of said demodulator means and applying said potential to the other of said signal amplifying channels to suppress the flow of signals therethrough in response to an increase in the strength of signals applied to said demodulator means, automatic volume controlv meansl comprising a Vdiode rectifier `connected with one of said channels to receive signals therefrom to be rectified and having an output resistor for rectified signals, means for applying automatic volume control potentials from said output resistor to said receiving Vsystem preceding said Vchannels to reduce the gain therethrough in response to an increase in the amplitude of received signals, means for establishing onY said output resistor a xed biasing potential, and means for varying said potential, thereby to vary the gain of said receiving system independently of the automatic volume control means. Q

2. A radio receiving system comprising, in combination, signal receiving means, means vproviding a signal amplifying channel, means providing a second signal amplifying channel having a relatively broad frequency response characteristic and a control channel for said rst and second named channels comprising an amplifier stage for said received signals and a plurality of rectier devices 'connected With said amplier stage to receive the amplified signal output therefrom, means for controlling the gain of said first named signal amplifying channel inversely in response to variations in the strength of a received carrier wave, means for controlling the gain in the broadly responsive signal amplifying channel directly in response to variations in the strength of a received carrier Wave, means for suppressing the iiow of signals through said broadly 'responsive signal amplifying channel connecte-d with another of said rectifying .devices to receive rectified signal output potential therefrom, and means for applying a variable delay potential to said last named rectifier device, whereby said suppressorv is Vrendered ineffective above av predetermined signal strength. i

3. A radio receiver in Aaccordance with claim 2 further characterized by the fact that means are provided for applying to each of said diode rectifier devices a predetermined differing delay potential, certain of said potentials being variable, whereby the response of said amplifying channel to the received signal may be controlled and the change from one amplifying channel to the other may be adjusted for a predetermined signal strength.

4. In a radio receiving system having three intermediate frequency amplifying channels, the combination of means in two of said channels providing differing frequency band width response and gain therein, and means in connection with the third amplifying channel for diferentally controlling the signal l amplifying operation of said two channels` in response to variations in signal strength to bring one channel into operation and to cut off operation of the other channel, and means in connection with said third channel control means for adjusting a control potential therein whereby the signal strength for determining the change from one amplifying channel to the other may be adjusted.

5. In a radio receiving system having a plurality of signal amplifying channels, the combination therewith .of a control channel including signal amplifying means and automatic volume control means for said first named amplifying channels, means for controlling said first named channels in response to signal variations whereby one of said channels at a time is effective to amplify signals, and means in said control channel for determining the point of change from one channel to the other and the signal amplitude required to effect such change.

6. In a superheterodyne radio receiver, the combination with a pair of intermediate frequency amplifier channels, one of which has a relatively broad and the other of which has a relatively sharp frequency response characteristic,

Y' of a third intermediate frequencyv amplifying channel including automatic volume control means for l said receiving system, suppressor means for the relatively broad channel included vin said channel, and suppressor means in the broad amplifying channel for controlling the re1- quency amplifier providing a plurality of intermediate frequency amplifying channels of differing frequency band width and gain, means providing a separate control channel including an intermediate frequency amplifier device and a plurality of diode rectifier devices for amplified signals fromsaid device, means responsive to received signals of predetermined amplitude connected with said control channel for suppressing the flow of signals through an intermediate frequency amplifying channel having Ya wide frequency'band width, and means for gradually suppressing the flow-'of signals through another of said channels inrresponse to the flow of received signals through said first named channel when receiving Irelatively strong signals.

8. In a radio receiving system, the combination as defined in claim 7 further characterized by the fact that` means are provided forcontrolling the change from one intermediate frequency amplifying channel to another in response to variations in the strength of a received carrier Y wave, whereby the audio frequency output from said signal amplifying channels is substantially constant.

9. In a radio receiving system the combination of means providing a pair of signal amplifying channels one of said channels being more broadly responsive to received signals than the other of said channels, means responsive to signal strength variations for controlling the sensitivity of said channels in opposite sense to render the more broadly responsive channel effective toamplify signals only in response rto signals of predetermined high amplitude,` and means for limiting the more sharply responsive signal amplifying channel for amplifying signals of a lower amplitude,

said signal responsive controlling means including an adjustable control element and a diode biased amplifier tube having an anode circuit connected with said more broadly responsive signal channel for adjustably controlling the point of changeover from signal reception in one channel to signal reception in the other channel.

10. In a radio receiving system the combination of a pair of parallel connected signal amplifying channels each comprising a gain controllable ampliiier and one of said channels providing a higher degree of amplification and being more sharply responsive toreceived signals than the other of said channels, automatic volume control means including a diode signal rectifierc'onnected with said sharply responsive channel amplifier for controlling the gain therethrough inversely in response to signal strength variations, a second diode automatic Volume control means including a second diode rectifier connected with the more broadly responsive signal channel amplifier for controlling the gain therethrough,inversely'in response to signal strength variations and an electric discharge amplifier device having an anode circuit connected in-common with said last named diode to apply a controlling potential to the more broadly responsive signal amplifier thereby normally to suppress signal flow therethrough, a third diode rectifier device connected with said electric discharge amplifier device to apply a biasing potential thereto in the presence of signals thereby to remove the suppression control from said more broadly responsive signal channel, and means for applying to said last named diode rectifier device an adjustable delay potential -whereby said suppression is removed in response to signals of a predetermined amplitude.

11. A radio receiving system in accordance with claim l0 further characterized by the fact that the more broadly responsive signal amplifying 75 channel includes a fourth diode rectifier device providing a control potential responsive to signal strength variations following the r-emoval of suppression from said channel, and means for applying said potential to an amplifier device in said more sharply responsive signal amplifying channel in a direction to cause the gain therethrough gradually to be reduced in response to increased signal strength through said broadly responsive signal channel thereby to suppress signals through said more sharply responsive signal amplifying channels.

12. In a radio receiving system the combination of means providing a plurality of parallel connected signal amplifying chanels one of which terminates in a plurality of diode rectifier devices each responsive to received signals andY having separate output circuits, means for applying differing delay potentials to said rectier devices thereby to control the point of response of said rectier devices to received signals,v means for applying a controlling potential responsive to signal strength variations from the output circuit of each of said diode rectier devices to one of said signal amplifying channels, means providing a noise suppressor` control for one of said signal amplifying channels having a control potential source responsive to signal strength variations in the other of said channels, means providing aY second noise suppressor control system in connection with one of said first named diode rectier devices for said last named signal amplifying channel, and means in said suppressor control system providing a variable delay potential for the .diode rectier device associated therewith whereby said last named signal amplifying channel is caused to be responsive to signals having a signal amplitude above a predetermined amplitude.

13. In a radio receiving system in accordance with claim 12, further characterized by the fact that said signal channels are' terminated by diode rectifier devices and a resistor network providing a common output circuit with said diode rectier devices connected substantially in parallel relation to each other thereby to provide signal output and biasing potentials for said output circuit and that the automatic volume control circuit for one of said diode rectifier devices connected with the more sharply responsive signal amplifying channel is provided with an adjustable source of biasing potential for the output circuit thereof whereby the sensitivity of the receiving system may be adjusted manually.

POUL F. G. HOLST. LOREN R. KIRKWOOD.` 

